LIGHT-SCATTERING STUDY OF PHASE-TRANSITIONS IN AQUEOUS-SOLUTIONS OF NONIONIC AMPHIPHILES

The dynamical behavior of water in aqueous solutions of the nonionic polyoxyethylene surfactant C10H21(OCH2CH2)(5)OH has been studied using Raman and depolarized Rayleigh-wing scattering, with particular attention to changes due to the structural phase transitions apparent at relatively large amphiphile concentrations. The frequencies and intensities of the two principal components evident in Raman spectra of the water O-H stretching band change at the hexagonal to isotropic phase transition. The width and intensity of the single resolvable line in the depolarized Rayleigh spectra similarly change at this transition: the relaxation time of the rotational water mode increases in the hexagonal phase. The changes are all consistent with an increase in the hexagonal phase of the proportion of water bound to the oxyethylene head groups: the environment of water in the system differs significantly from that in bulk water. At the lamellar to isotropic phase transition very much smaller changes were apparent, but only in the Raman scattering data. This lesser sensitivity suggests that in the lamellar phase almost all of the water is bound to the amphiphile head groups, as in the isotropic phase at such volume fractions. The changes in water dynamics reflect the structural changes in the solutions at the phase transitions, in particular the differing packing constraints in the various phases.